In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the cardiac chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles, via the atrioventricular (A-V) node and a ventricular conduction system, causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate.
A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
Early pacemakers delivered stimulation pulses to the heart at a fixed rate. Fixed rate pacing, however, is not physiological in that periods of increased activity or metabolic demand are not accompanied by the normal physiological rise in heart rate. Rate-responsive pacemakers were, therefore, introduced. Rate-responsive pacemakers employ sensors that indicate changes in physical activity or metabolic demand.
One commonly used sensor is an accelerometer that is positioned within the housing of the stimulation device that is implanted within the patient's body. As the patient becomes more active, the accelerometer measures the resulting acceleration and provides an activity signal that is indicative of the increased acceleration experienced by the patient.
Activity sensors of this type are generally thought to provide a good indication of the metabolic demand of the patient for newly initiated, brisk, low-level activity. In other words, when the patient initiates a new brisk, low-level activity, such as walking and the like, the accelerometer in the activity sensor provides a good indication of the sudden increase in the level of activity of the patient which generally results in heightened metabolic demand requiring the heart to deliver more oxygenated blood.
While activity sensors of this type are adequate for providing an indication of the onset of brisk, low-level activity, these sensors have several shortcomings. For example, the signal that is often provided by such activity sensors becomes blunted when the patient is engaged in high exertion exercise. In other words, when the patient is heavily engaged in a particular physical activity, the activity signal may not provide a sufficient indication to the control unit of the need for more oxygenated blood as a result of the increased activity. As an illustration, the output signal from a typical activity signal can be inaccurate for assessing the patient's actual metabolic need when the patient is performing an action like carrying a heavy object.
The degree of acceleration detected by the activity sensor is likely to correspond to a perceived low metabolic demand activity, such as walking, and would not account for the increase in metabolic demand as a result of carrying the heavy object. Moreover, acceleration-based activity sensors are also subject to providing false readings as a result of the patient experiencing accelerations that are unrelated to physical activity, such as, for example, the patient traveling on a bumpy road in a vehicle.
Other types of physiologic sensors may be used to provide an indication of metabolic demand. One common type of physiologic sensor is a minute ventilation sensor that measures the respiration rate and tidal volume of the patient's respiration. Respiration is normally related to metabolic demand. Therefore, the rate at which a patient is breathing and the volume of air being breathed is normally indicative of the metabolic demand of the patient.
One typical way of obtaining a minute ventilation signal is to periodically measure the transthoracic impedance between a lead implanted within the patient's heart and an indifferent electrode, such as the housing of the implanted stimulation device. As the transthoracic impedance is proportional to the chest volume, measuring this particular impedance value provides an indication as to the degree to which the patient's chest is expanding and contracting and the rate at which such expansion and contraction is occurring. The greater the patient's breathing rate and the greater the tidal volume of the breaths, the more likely it is that the patient has a heightened need for delivery of oxygenated blood by the heart.
While physiologic sensors, such as minute ventilation sensors, provide a strong indication of the metabolic demand of the patient, these sensors also have several disadvantages for use in determining the needed cardiac stimulation rate. In particular, the values provided by these sensors often lag in time behind the actual metabolic demand of the patient. Consequently, these sensors are typically not particularly well suited for providing the sole indication of the actual metabolic demand of the patient when the patient is initiating or ceasing physical exertion.
To address the problems associated with both of these types of sensors, rate responsive pacing systems have been developed which utilize the signals from two or more types of sensors to determine a desired heart stimulation rate.
One goal of dual or multi-sensor rate response devices is to provide the most normal sinus response to changes in activity and metabolic demand possible by determining the rate response based on two or more sensor indicated rates. Differences in sensor indicated rates do exist due to different response times of the sensors or different sensitivities to a particular form of activity or exertion. Therefore, algorithms for determining a stimulation rate adjustment based on two or more sensors may include assigning various weighting factors to received sensor signals or applying different processing parameters to sensor signals (e.g., filtering).
Differences between sensor indicated rates (or sensor levels) provided by multiple sensors may also arise when an abnormal patient condition exists. For example, Cheyne-Stokes respiration, which is the waxing and waning of breathing, is known to occur and gradually worsen in heart failure patients. During Cheyne-Stokes respiration, a minute ventilation sensor will sense a high minute ventilation, even during rest, followed by a period of very low minute ventilation. The high minute ventilation during an episode of Cheyne-Stokes respiration would falsely indicate a high metabolic need and therefore an unnecessarily high sensor indicated rate. Thus, a minute ventilation sensor may be reflecting periods when respiration is abnormal rather than when an actual change in metabolic demand occurs.
A discrepancy between individual sensor indicated rates (or sensor levels) may arise when one sensor is functioning abnormally, e.g., responding to extraneous noise, or when the sensor indicated rate calculations are programmed to be either too sensitive or too insensitive to sensor signal changes. When programmed optimally, the sensor indicated rate used to adjust stimulation rate will be determined from each sensor some of the time.
If the rate is adjusted in response to one sensor, all or a majority of the time, the benefit of dual or multi-sensor rate determination may be lost. One sensor may be programmed such that the sensor indicated rate is too sensitive to sensor signal changes, or another sensor may be programmed such that the sensor indicated rate is not sensitive enough to sensor signal changes.
It would be desirable to provide a cardiac stimulation system and method for monitoring sensor indicated rate discrepancies such that the frequency and probable cause of such discrepancies can be understood. Such monitoring would allow abnormal patient conditions to be tracked or adjustments to operating parameters controlling the rate response based on sensor indicated rates to be optimized.
To this end, a method is needed for detecting discrepancies in sensor indicated rates and recording the occurrence of such discrepancies. It is also desirable, in a dual or multi-sensor, rate-responsive stimulation device, to provide a method for documenting sensor indicated rate differences so that the programming of operating parameters used in calculating sensor indicated rates can be optimized.